Delta cases may finally be reaching a peak in the US. Future surges are likely to be weaker.
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- The US's Delta cases are rising at a slower rate than earlier in the summer.
- In countries like the UK and India, Delta surges seemed to spike quickly then fizzle.
- Experts say the US is still vulnerable to more Delta outbreaks, but future surges will likely be weaker.
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The Delta variant appears to be loosening its hold on the US.
Roughly two months into this fourth case wave, the growth rate of daily cases is slowing, suggesting that a peak is nigh. Nationally, daily cases rose 9% in the last week, and 8% the week prior. That's compared to double-digit weekly increases throughout July and early August - and a 61% increase from July 21 to 28.
Over the last several weeks, daily cases have declined in southern states hit hardest by Delta, including Arkansas, Missouri, Louisiana, and Florida.
A model from the University of Washington's Institute for Health Metrics and Evaluation suggests the US's true number of new daily infections may have peaked in mid-August, since official tallies usually represent infections contracted two weeks prior. Under the IHME's current projection, infections could steadily decline for the rest of the year.
"I do think that we're going to see the Delta surge wane, like others have," Jeffrey Morris, director of biostatistics at the University of Pennsylvania, told Insider.
At the very least, Morris said, daily hospitalizations could decline soon, given how much of the population has at least some form of immunity: 62% of Americans are at least partially vaccinated and 12% have had a confirmed case of COVID-19, though these groups overlap.
"It's possible we might see [outbreaks] pop up in other parts of the country that haven't been hit hard yet" by Delta, Morris said, but he added that "unless we have another, even more transmissible variant, we shouldn't expect any future surge to be as intense as this one."
Delta surges seemed to spike quickly then fizzle in other countries
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If Delta cases do drop off soon in the US, they would follow a pattern seen in several other countries: The variant appears to tear through a population like wildfire, then fizzle out fairly quickly.
India's Delta outbreak started in early March, then peaked roughly two months later. The UK's outbreak also peaked after two months, with cases rising from late May until late July. Delta surges in France and Indonesia were even shorter: They peaked after around one-and-a-half months.
By comparison, it took around four months for coronavirus cases to reach their peak during the US's third wave of infections over the fall and winter.
One likely explanation for Delta's sharp rise and fall, Morris said, is that the variant is highly transmissible, but fewer people are susceptible to infection now than at any other point in the pandemic.
"People with no immune protection - no vaccination or previous infection - they're kind of like dry kindling," he said. "If a spark hits it, it's going to light on fire. Whereas the more immune protection an individual has from the previous infection, vaccination, or both, then they're kind of like a wet log. If you have a big enough, hot enough fire, the wet log will burn, but not as much."
Could cold weather and in-person school drive cases up again?
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But if the pandemic has taught us anything, it's that no pattern is guaranteed.
As the weather gets cold, people will inevitably spend more time indoors, where it's easier for Delta to spread. Some experts think the US hasn't seen the worst of school transmission, either, since some schools don't resume in-person class until later this month.
"The folks that kids, teachers, and parents were interacting with over the summer aren't the same folks as who they'll be interacting with in the fall," Dr. Maimuna Majumder, a research associate at Harvard Medical School, told Insider. "Mobility patterns shift, too, as college students fly from their home cities and countries to go back to university in-person. Both of these realities carry with them the chance to 'stir the pot.'"
She added: "I'm not sure that the slight plateau that we're seeing nationally will last."
But Morris said school transmission usually isn't enough to drive up national cases on its own.
"It would be reasonable to expect there might be an uptick from school going back - whether it really lights a new forest fire, I personally don't think so," he said.
Plus, the risk of transmission goes down as vaccination rates rise, and the US's daily vaccinations have been steadily climbing over the last two months. Daily vaccinations rose 20%, on average, from the end of July to the end of August.
"The vaccination is a little more in our favor, but Delta still spreads like crazy," Morris said. "We'll see what people's behavior is like."
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